(1) Field of the Invention
The present invention relates generally to a system for establishing the sound velocity profile of a medium, and more particularly to a cable for use in such a system.
(2) Description of the Prior Art
Undersea cables containing optical fibers are well known in the art. U.S. Pat. No. 5,125,062 to Marlier et al. relates to an undersea telecommunications cable having optical fibers. The undersea cable has an optical fiber embedded in material filling a tube which itself lies inside a helical lay of metal wires having high mechanical strength, the interstices between the wires of the helical lay being filled with a sealing material. The helical lay is surrounded by an extruded sheath made of an electrically insulating and abrasion resistant material, and for the purposes of remotely powering equipment interposed on the cable, the cable includes conductive means either belonging to the helical lay or surrounding it, which conductor means is surrounded by the sheath.
U.S. Pat. No. 4,971,420 to Smith relates to an optical fiber cable for submarine use which has a core surrounded by a layer of strength members which include both wires and laser welded metallic tubes containing the optical fibers.
U.S. Pat. No. 5,212,755 to Holmberg relates to an armored fiber optic cable having both fiber optics and armor wires located outside the cable core in position where the fiber optics experience low strain when the cable is under stress. In one embodiment, metal armor wires and optical fibers embedded in metal tubes are arrayed in one or more layers about and outside the cable core. In another embodiment, KEVLAR armor wires and optical fibers embedded within a hard composite shell are arrayed in one or more layers about and outside the cable core, and a layer of KEVLAR armor is provided surrounding the one or more layers. In each of the embodiments, the strains that the fiber optics experience due to core stresses and due to core residual strain is materially reduced over other armored fiber optic cables.
U.S. Pat. No. 5,495,547 to Rafie et al. is directed to a well logging cable including first conductor elements, each of the first elements consisting of a steel wire surrounded by copper strands and covered in an electrically insulating material, and at least one second conductor element including at least one optical fiber enclosed in a metal tube, copper strands surrounding the tube and strands covered by the electrically insulating material. The first elements and the at least one second element are arranged in a central bundle. The second conductor element is positioned within the bundle so as to be helically wound around a central axis of the bundle. The bundle is surrounded by armor wires helically wound externally to the bundle.
The velocity of sound through a medium depends upon a number of factors including temperature, pressure and density. In the case where the medium is seawater, sound velocity also depends on the salinity of the seawater. In many situations, it is necessary to obtain accurate measurements of sound velocity through a medium along an axis, such as obtaining a profile of sound velocity of a water column. For example, sound velocity measurements or profiles are needed for accurate sonar location of objects on the sea bottom in recovery operations or for accurate bottom mapping.
U.S. Pat. No. 5,734,623 to Ruffa illustrates a fiber optic cable, coated to increase its sensitivity to acoustic pressure, which may be towed through a medium. The optical fiber contains Bragg grating sensors at regular intervals along its length. A steerable array of transducers sends a pulse of sound in the direction of the optical cable while broadband pulses of light are directed down the optical fiber. The pulses of light are selectively reflected back according to the spacing between the Bragg gratings. The sound pressure field causes a local strain in the fiber, thus changing the wavelength of the grating. The sound velocity profile along the length of the optical cable is computed by measuring the amount of time necessary for successive Bragg gratings to respond to the acoustic pressure associated with the advancing wave front of the acoustic pulse.
Although an instrumented tow cable that continuously measures the sound velocity profile has the potential to significantly improve sonar performance, it has not yet been realized in fleet sonar systems. One of the main obstacles is to design such a system that is sufficiently rugged to survive deployment and retrieval through handling systems at high speeds which lead to high tensions. This requirement alone rules out attaching devices to the cable or embedding devices into the protective jacket surrounding the cable such as thermisters to measure the temperature profile of the ocean. For this reasons, fiber optic Bragg grating-based sensors are ideal, since they require no wires or preamps that can be crushed; the fiber is the sensor.